Interpretive Summary: Methionine is an important nutrient in poultry diets. Corn with increased methionine levels would allow poultry diets to be formulated for less cost. We used traditional breeding methods to develop corn with unusually high or low levels of methionine. By examining these corn varieties at the molecular level, we were able to gain insight into how the plant controls the level of methionine in the grain. We identified specific genes that are involved in controlling methionine levels, and propose a model to explain the regulation of grain methionine levels. This information will make it easier to develop new corn varieties with increased levels of methionine. Ultimately, this work will benefit the consumer by reducing the cost of meat and egg production.

Technical Abstract:
Methionine is an important nutrient in animal feed and several approaches have been developed to increase methionine concentration in maize (Zea mays L.) grain. One approach is through traditional breeding using recurrent selection. Two populations selected were selected for high and low methionine concentration for eight generations and 40% and 56% differences between the high and low populations in grain methionine concentration were observed. Mean values between the high and low methionine populations differed by greater than 1.5 standard deviations in some cycles of selection. Other amino acids and total protein concentration exhibited much smaller changes. In an effort to understand the molecular mechanisms that contribute to these differences, we compared transcript levels of candidate genes encoding high methionine seed storage proteins involved in sulfur assimilation or methionine biosynthesis. In combination, we also explored the genetic mechanisms at the SNP level through implementation of an association analysis. Significant differences in methionine-rich seed storage protein genes were observed in comparisons of high and low methionine populations, while transcripts of seed storage proteins lacking high levels of methionine were unchanged. Seed storage protein levels were consistent with transcript levels. Two genes involved in sulfur assimilation, Cys2 and CgS1 showed substantial differences in allele frequencies when two selected populations were compared to the starting populations. Major genes identified across cycles of selection by a high-stringency association analysis included dzs18, wx, dzs10, and zp27. We hypothesize that transcriptional changes alter sink strength by altering the levels of methionine-rich seed storage proteins. To meet the altered need for sulfur, a cysteine-rich seed storage protein is altered while sulfur assimilation and methionine biosynthesis throughput is changed by selection for certain alleles of Cys2 and CgS1.